Australia is an extensive, low continent with an area of about
7.66 million km2, and a mean height above the present sea
level of 330 m, and 200 km2 less than 200 m above sea level.
The highest point on the continent is Mt Kosciuszko that rises to 2,227
m, not an impressive height on a world scale. According to Twidale6
Australia is a compact continent that has few major islands, inlets or
embayments and a coastline of a bit less than 20,000 km. The ratio of
shore length to land area of about 1 km of coast for every 390 km2
(1:390), while the ratio of coast to land area of peninsula and insular
Europe is 1:75. The Gulf of Carpentaria, Bonaparte Gulf and King Sound
on the north coast, Exmouth Gulf and
Shark Bay on the west and Port
Phillip Bay and gulf St. Vincent and Spencer Gulf on the south. Tasmania
and Kangaroo Island in the south
Fraser Island on the east, and
Groote EyIandt, Melville Island and
Bathurst Island off the north
coast are the only major islands. Endorheic streams that flow to basins
of internal drainage serve more than half of the continental area. The
Lake Eyre catchment accounts for 1.3 million km2 of central
and northeastern Australia. Exoreic streams (rivers that flow to the
sea) serve less than half the continent, with the
River Murray being of
notable length but has a low discharge rate as for much of its length it
passes through arid areas.

The Australian continent is unique in a number
of ways. It is the driest, flattest, oldest, it has the most
erratic climate of any continent, and much of it experiences
great extremes of rainfall both during a year and from year to
year. As a result of this, poor soil, arid and unpredictable
climate over most of the continent, the plants, and therefore the
animals that depend on them, have evolved into a unique flora and
fauna.

Lack of volcanic or seismic activity leading to
mountain building over much of the continent for many millions of
years means that what mountain ranges there were have been worn
down by erosion to mere stubs of their former glory. The soil
over much of Australia, especially the dry central parts, have
not been renewed by volcanic activity for a very long time and
are impoverished compared to soils from most other parts of the
world. Another factor in the formation of fertile soils that was absent in
Australia was glaciers. During the ice ages of the last 2.6 million years,
glaciers covered much of the Northern Hemisphere, grinding unweathered rock into
fine particles that became soil enriched with new minerals when the glaciers
retreated. During this time period Australia had
cold windy
periods at times of Northern Hemisphere glaciations, but no, or very limited,
glaciers. Any that formed were on the high country that is such a small part of
the Australian landscape.

After being recycled many times over much of the history of
the Earth, with no new rock added by volcanism, the soils of arid Australia have been weathered and leached more than any others
making them the most nutrient-deficient soils in the world. And
most of Australia spent many millions of years covered by seas.
In many places these seas were eventually cut off from the ocean
and evaporation formed huge salt deposits which were later buried.
So added to this impoverished state of the soils there are vast areas where
there are salt deposits beneath the surface which means when land
is cleared for agriculture the removal of deep-rooted trees and
shrubs mean the water table rises. By the time the water table
gets close to the surface it has passed through these salt layers
and so causes salinisation in the root zone and eventually at
the surface, which makes the land useless for agriculture.

The geologic history and its climatic contrasts are reflected in the
landforms. There are 3 major structural components, the stable Western
Shield, the gently warped Central Basin, and the ancient orogony of the
Eastern Uplands, which have been rejuvenated by differential uplift in
Tertiary and later times. The result is the vast plains and plateaux of
the Australian landscape. There are not many areas above about 1700 m,
and even at Mt Kosciusko, the highest point on the continent, it is less
then 3000 m above sea level. Not high compared with the mountains in
other parts of the world.

According to Twidale & Campbell (Source 5), the lack of recent earth
movement is a main cause of the widespread prevalence of relatively
low-lying plains. The Late
Palaeozoic
was the time when the most recent episode of mountain building occurred
in Australia, with the deformation and uplift of the
Eastern
Highlands. There have been many episodes of warping and faulting,
that were widespread but minor, since the orogenesis that produced the
Eastern Highlands. The authors suggest that about 30 million years would
be required to base-level a continent of the size of Australia, after
allowing for isostatic compensation that would result from erosional
unloading, allowing plenty of time since the last major orogenesis for
the formation of extensive plains by weathering and erosion. According
to Twidale & Campbell, the compact shape of the Australian continent has
contributed to the tendency for widespread plantation, with few major
indentations or embayments along the extensive coastline. This meant
that as sea level changes occurred the resulting changes of river
behaviour were limited to the margins of the continent, with the coastal
rivers being the only rivers affected, the inland rivers being shielded
from such impacts. On the east coast the short, steep rivers have not
regressed as much as would be expected, some having eroded headward by
about 100 km in about 60 My. In the later Tertiary the
Lake Eyre
Basin began to subside, resulting in the internal (endogenetic) drainage
system that drains much of inland Australia. Depositional plains are
also a prominent feature of inland Australia (Twidale & Campbell, Source
5).

These components have determined the outline of the continent and the
overall pattern of drainage and relief. In the east, the peripheral
uplift in the Eastern Uplands has resulted in the highest ground being
near the coast.

In the west of the continent, there is a narrow plain
between the faulted edge of the shield and the coast. In the north-west
the highest ground is in the marginal line of the
Hamersley,
Kimberley
and
Arnhem Land Plateaux. The external drainage on 3 sides of the
continent is restricted to a narrow strip around the edge of the
continent on 3 sides totaling about 1/3 of the continental area.

The Central Basin has 2 major inward-draining drainage systems, the
interior drainage towards lake Eyre, and the
Murray-Darling system. The
Murray-Darling system has maintained a connection to the sea, mostly
because of the extra water draining from the south-eastern part of the
Great Dividing Range.

The structure of the
drainage systems, where the coastal areas, where
most of the rain falls, is drained to the sea, and the inward flowing
drainage of the rest of the continent, that receives very little rain,
conspires in such a way that vast tracts of central Australia depend for
their water on the intermittent floods that are channeled from the
monsoonal areas of northern Australia along normally dry channels. While
the monsoon rarely fails completely, the amount of rain delivered to the
north varies. It is only in very wet years that the water reaches all
the way to Lake Eyre, especially as a lot of the water is absorbed into
the dry channel beds and evaporates in the hot dry air of the
interior long before it reaches
Lake Eyre.
Away from the ameliorating influence of the coast the temperatures can
get very high in central Australia during summer.

Nearly all of the area inland of the peripheral drainage systems
generates less the about 3 cm per year of run-off. As aridity has
increased any rivers that had existed on the low gradient sandy surfaces
of the shield areas have disintegrated or disappeared entirely. A
similar condition, no rivers, occurs also on the
Nullarbor Plain and the in the
Simpson Desert.

These 3 major structural components are convenient divisions for
describing the landform assemblages of Australia.